Apple Replant: Treating the Cause, Not the Symptoms

Replant diseases didn't just recently rear their ugly heads. Growers have been battling them in various crops since the 17th century, according to Mark Mazzola, a plant pathologist with USDA's Agricultural Research Service (ARS).

"Because no one has fully understood what causes these disorders, growers for several decades have been controlling them with broad-spectrum soil fumigants. Many growers in the United States control replant problems with methyl bromide," Mazzola says. He is with the ARS Tree Fruit Research Laboratory in Wenatchee, Washington.

But methyl bromide will no longer be available to growers after January 1, 2001. Other broad-spectrum biocides—like metam sodium, Telone, and chloropicrin—have been suggested as replacements for methyl bromide. Each of these compounds is associated with potential problems ranging from groundwater contamination to health concerns, Mazzola reports.

Mazzola has discovered that a fungal complex causes replant disease in Washington's apple orchards. Also, he has found beneficial bacteria that suppress the disease in apples and has filed a patent application for their use as biocontrol agents.

What Is Replant Disease?

Replant disease is often the major problem for growers in establishing an economically viable orchard on a site where apples had been grown previously. And apples are big business. The primary tree fruit production region in North America is in the western United States. Here apples are grown on more than 200,000 acres, accounting for 65 percent of total U.S. production. Washington State alone grows about 180,000 acres of apples. This constitutes about 50 percent of U.S. apple production and generates over $1 billion in farm income. Failure to control apple replant in Washington can result in a $40,000-per-acre reduction in gross returns over 10 years. Each year, growers in Washington replant about 10,000 acres of apples.

Not only do growers incur tree replacement costs in affected orchards, but trees struck by the disease begin bearing fruit 2 to 3 years later than normal, and their yields don't compare to those achieved in the absence of the disease.

"Replant disease of apple was thought to occur primarily at sites where growers replanted after removing very old fruit trees. However, we have documented the onset of the fungal complex within 3 years of establishing the orchard on new ground. Also, the disease attacks nurseries where previous plantings of apples were grown for 1 to 2 years," Mazzola says. "When apple replant affects an orchard, young trees grow unevenly, and eventually, because of disease pressure, growth of most of the trees in the orchards will decline. Trees can also die."

Severe stunting, shortened internodes, rosetted leaves, and reduced fruit production are all symptoms of apple replant disorder, he says. Affected trees have small root systems with lots of fibrous roots that don't function properly because of decay caused by the disorder.

What Causes Apple Replant?

"Although apple replant has been attributed to a variety of biotic and abiotic factors, other types of fruit trees grow normally in the same soil," Mazzola reports. "And pasteurizing or fumigating that same soil dramatically increases growth. This proves that the disease is primarily a biological phenomenon."

Numerous soil- and plant-associated microorganism have been implicated as causal agents of apple replant disease, including the lesion nematode (Pratylenchus spp.) However, "our research data don't indicate that this nematode has a role in apple replant disease in Washington," Mazzola reports.

In a recent study in nine commercial and experimental orchards in Washington, Mazzola's data clearly showed that several species of fungi from the genera Cylindrocarpon, Phytophthora, Pythium, and Rhizoctonia are the primary causes of apple replant disease.

"Surprisingly, the components of this complex group of fungi that causes the disease were consistent among the experimental orchards, although the relative contribution of these fungi to disease development varied between orchards," Mazzola notes. "Maybe even more importantly, these studies showed that nematodes and bacteria have a very limited, if any, role in disease development."

Now that Mazzola has identified the fungal complex responsible for replant disease, he and his colleagues are focusing on biological, cultural, and narrow-spectrum biocides which target individual causal agents within that complex. They have focused current studies on developing an integrated system to manage apple replant disease.

How Do You Treat the Disorder?

"Now that we know just what causes the problem, we can target the individual components of the group," Mazzola says. "In the past, cultural and biological measures as fumigant alternatives failed under field conditions."

In greenhouse studies, Mazzola and colleagues eliminated microbial agents with semiselective biocides and soil pasteurization. In four of five replant soils tested, the fungicides difenconozole and metalaxyl spurred growth of apple; fludioxinil worked in two soils tested.

Mazzola conducted an initial field trial of these fungicides in 1997 and found that, individually, the fungicides improved growth up to 40 percent. He has expanded the field trials to evaluate fungicide combinations.

"In an ongoing study, pasteurizing the soil in a replant-affected orchard caused significant improvements in plant growth. In fact, growth was equivalent to that obtained in previously unplanted soil from the same site," Mazzola reports.

Mazzola has found two bacterial organisms that show promise in controlling apple replant disease in the greenhouse. "We are still in the early stages of our evaluation trials and need additional trials of the organisms under field conditions. David Granatstein of Washington State University, is working with us to test the organisms in commercial orchards."

Mazzola is also working with Andre Levesque, Agriculture and Agri-Foods Canada, Summerland, British Columbia, on seasonal fluctuations in the species composition of Pythium populations. Pythium is one of the fungal agents that cause replant disease in apple.

"We've tentatively seen resistance to metalaxyl in one Pythium population and will conduct a more in-depth study," Mazzola says.

In seeking different alternatives, Mazzola and colleagues have made some interesting discoveries about rotating wheat in replant soil.

"We found that planting three short-term (3 weeks) cycles of wheat in replant soil significantly increased the growth of apple seedlings. We tested spring, winter, and club types of wheat, but strangely enough, we got these results only with Eltan, a soft, white, winter wheat variety," he reports. "We also noticed that the soil planted to Eltan suppressed Rhizoctonia root rot of apple caused by Rhizoctonia solani AG–5.

Mazzola plans more research on using wheat as a cover crop. He planted an initial field trial this season.

Last Updated: July 24, 1998